I. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method and apparatus for performing conducting telephone calls or other communications between wireless telecommunications systems that supports inter-system soft handoff.
II. Description of the Related Art
FIG. 1 is a diagram of a cellular telephone system configured in accordance with the use of the IS-95 over-the-air interface standard. The IS-95 CDMA over the air interface standard (IS-95) promulgated by the Telecommunications Industry Association (TIA) defines a protocol for providing wireless telephone service via code division multiple access (CDMA) modulated RF signals. IS-95 has many derivatives and related standards including IS-95-A, ANSI J-STD-008, IS-99 and IS-657, referred to herein collectively as "IS-95." A cellular telephone system configured substantially in accordance with the use of IS-95 is described in U.S. Pat. No. 5,103,459 entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System" assigned to the assignee of the present invention and incorporated herein by reference.
During operation, subscriber units 10 (typically cellular telephones) conduct communications by interfacing with base stations 12 using CDMA modulated RF signals. From base stations 12 the communications are further conducted using base stations controllers (BSC's) 14 and mobile switching centers (MSC's) 16 with public switched telephone network (PSTN) or other subscriber units 10. BSC's 14 provide call mobility functionality, as described in greater detail below, and MSC's 16 provides call routing, billing and switch interface functionality. Additionally, the data exchanged between the various systems shown in FIG. 1, including subscriber units 10 and base stations 12, are processed in frames. This data can be either traffic data which includes voice information, or signaling data which is used to configure and control the various systems shown.
The use of CDMA technology allows adjacent base stations in an IS-95 system to conduct communications over the same RF bandwidth, which increases the frequency reuse factor of the cellular telephone system when compared to other types of cellular telephone technology. When combined with extensive transmit power control, increasing the frequency reuse factor increases the efficiency with which the cellular telephone system uses the available RF bandwidth, which is one of the advantages of the IS-95 standard.
Another advantage of allowing the same RF bandwidth to be used by adjacent base stations 12 is that "soft handoff" can be used to transition a subscriber unit 10 from the coverage area of a first base station 12 to the coverage area of a second base station 12. Soft handoff involves simultaneously interfacing a subscriber unit 10 with two or more base stations as the subscriber unit transitions between the associated coverage areas of the two base stations 12. One of the subscriber units 10 shown in FIG. 1 is in soft handoff.
Soft handoff can be contrasted with hard handoff, where the interface with the first base station 12 is terminated before the link with the second base station has been established. Maintaining at least one link at all times improves the reliability and quality of the communication being conducted. A system and method for performing soft handoff is described in US described in U.S. Pat. No. 5,101,501 entitled "Method and System for Providing a Soft Handoff in Communications in a CDMA Cellular Telephone System" and U.S. Pat. No. 5,267,261 entitled "Mobile Station Assisted Soft Handoff in a CDMA Cellular Communications System" both assigned to the assignee of the present invention and incorporated herein by reference.
As noted above, BSC's 14 perform various functions that allow mobile communications to be conducted including functions that facilitate soft handoff. In particular, BSC's 14 perform "frame selection" and "frame distribution" for the set of base stations 12 involved in the soft handoff call. Frame selection is the selection of one frame for further processing from the set of frames received from the set of base stations used during a soft handoff. Frame selection maintains the highest quality link by selecting the best frame available at all times.
Frame distribution is the duplication and distribution of frames directed to the subscriber unit 10 to the set of base stations 12 involved in the call. Frame distribution allows each base station 12 to transmit a copy of the frame to the subscriber unit 10 increasing the probability of successful reception of at least one frame.
As shown in FIG. 1, subscriber unit 10A is interfacing with a single base stations 12A and is located near the coverage area of a second base station 12B. Base stations 12A and 12B, however, are coupled to different BSC's 14. This complicates the process of performing soft handoff because neither BSC 14 has access to both base stations 12A and 12B. Performing soft handoff between two base stations 12 that are controlled by two different BSC's 14 is referred to as "inter-system soft handoff." Inter-system soft handoff can be contrasted with "intra-system soft handoff" in which a soft handoff is conducted between two base stations 12 controlled by the same BSC.
For inter-system soft handoff, frame selection cannot be performed as easily because neither BSC 14 has access to the set of frames that are produced by the two base stations 12. Additionally, frame distribution is also rendered more difficult because neither BSC can transmit the set of frames generated to both base stations. Nonetheless, it is necessary to use two or more BSC's 14 in many large metropolitan areas because the number of base stations required to provide adequate coverage exceeds the capabilities of a single cellular system formed using a single BSC 14, which in turn creates the need to conduct inter-system soft handoff.
A system and method for performing an inter-system soft handoff is described in copending U.S. patent application Ser. No. 08/649,959 entitled "Inter-system Calling Supporting Inter-System Soft Handoff" assigned to the assignee of the present invention and incorporated herein by reference. In this application, a channel between the two BSC's 14 is established via an interconnect for exchanging the data necessary to perform soft handoff. Additionally, each BSC is required to either store or obtain information typically tracked within an MSC, and therefore must interact with an MSC in order to perform the soft handoff. Preferably, the interconnect is comprised of a standard T-1 or E-1 link, which are well known and therefore relatively inexpensive to implement. To establish the channel, signaling messages are exchanged between the two BSC's 14 involved in the call. As noted above, the information contained in the signaling messages require each BSC 14 to track certain information including the number of calls being conducted over the interconnect, or interact with an MSC tracking that information, and to provide the other BSC 14 with certain predefined types of information. The number of calls on the interconnect are tracked to avoid overloading the interconnect.
Requiring BSC's 14 to obtain, track or provide certain types of information, however, is highly restrictive, and can be difficult for some manufacturers of BSC's to implement. Additionally, it requires a significant degree of coordination and cooperation between BSC and MSC manufactures, which is not always feasible. Thus, a system and method of performing inter-system soft handoff that requires less cooperation and coordination between BSC's 14 and MSC's 16 would be highly desirable.